GST compulsory registration for deductee u/s 51.

As per Section 24 of the GST Act, the following categories of persons shall be required
to be registered compulsorily irrespective of the threshold limit:
(a) persons making any inter-State taxable supply; 

It is important to mention that Central Government vide Notification No. 07/2017-Integrated Tax, dated. 14-09-2017 has provided that the job workers engaged in making inter-State supply of services to a registered person are exempted from obtaining registration [However, such exemption is not available to a job-worker
who is liable to be registered u/s 22(1) i.e. threshold limit or who opts for
voluntarily u/s 25(3); or of section 25 of the said Act; or job worker is involved in
making supply of services in relation to the goods mentioned against serial
number 151 in the Annexure to Rule 138 of the CGST Rules, 2017 i.e. jewellery,
goldsmiths’ and silversmiths’ wares as covered under Chapter 71 which do not
require e-way bill.]
Further, the Central Government vide Notification No. 10/2017 – Integrated Tax
dated 13-10-2017 has exempted the persons making inter-State supplies of
taxable services and having an aggregate turnover not exceeding an amount of `
20 lakhs in a financial year from obtaining registration / ` 10 lakhs in case of the
States specified in Article 279A (4) (g) of the Constitution except Jammu &
Kashmir [i.e., Exemption from obtaining compulsory registration u/s 24 of CGST
Act, 2017 in case of Inter State Supply of Services]
(b) casual taxable persons making taxable supply;
Please note that in terms of Notification No. 32/2017-Central Tax, dated 15-09-
2017 & Notification No. 8/2017 – Integrated Tax, dated 14-09-2017, the casual
taxable persons as well as the persons making inter-State taxable supplies of
handicraft goods as the category of persons exempted from obtaining registration
if the aggregate value of such supplies, to be computed on all India basis, does
not exceed an amount of ` 20 lakhs in a financial year (` 10 lakhs for Special
Category States other than J&K).
The casual taxable persons or the persons making inter-State taxable supplies, as the case may be, will be required to obtain a Permanent Account Number and generate an e-way bill in accordance with the provisions of rule 138 of the Central Goods and Services Tax Rules, 2017. Further, the list of eligible ‘handicraft goods’ and their respective HSN codes for the purpose of aforesaid notifications has also been provided for when they are made by the craftsmen predominantly by hand even though some machinery may also be used in the process.
(c) persons who are required to pay tax under reverse charge;
(d) non-resident taxable persons making taxable supply;
(e) an electronic commerce operator for whom the provision of section 9(5) of GST Act apply.
(f) persons who are required to deduct tax under section 51;
 

You are cover in clause in (f) . So you are required to compulsory registration